I had the same issue, afer debugging with firebug and crossing it with Apache logs, I found out that the plaugin was requesting the JS files using the HTTPS protocol prefix. After a short investiagtion, I found that in file ckeditor_class.php in line 30 there was:

<!DOCTYPE HTML PUBLIC “-//IETF//DTD HTML 2.0//EN”>
<html><head>
<title>500 Internal Server Error</title>
</head><body>
<h1>Internal Server Error</h1>
<p>The server encountered an internal error or
misconfiguration and was unable to complete
your request.</p>
<p>Please contact the server administrator,
root@localhost and inform them of the time the error occurred,
and anything you might have done that may have
caused the error.</p>
<p>More information about this error may be available
in the server error log.</p>
<hr>
<address>Apache/2.2.15 (CentOS) Server at www2.autani.local Port 80</address>
</body></html>